This invention relates to scanning type horizon sensors for indicating orientation by sensing a line of discontinuity in optical radiation between a reference planet and outer space, and more particularly, to such horizon sensors in which optical fibers are mounted in a standard motor shaft for driving the scanning mechanism of the sensor which transfers the focal point of radiation image thereon from one end of the motor shaft to the other.
Horizon sensors are devices which are mounted in satellites, space probes and the like which sense the horizon of a reference planet and produce a signal which indicates the orientation of the satellite carrying the sensor. Almost all unmanned satellites employ horizon sensors for attitude control and guidance. The attitude of the satellite is determined by its position with respect to three axes at right angles to each other, two of which are in a plane at right angles to a projected radius of the earth passing through the satellite and the third of which coincides with that radius. One way of deriving such information is by the use of a conical scan horizon sensor for example, of the type shown and described in U.S. Pat. No. 3,020,406 which is assigned to the Assignee of the present invention utilizing the horizon representing a line of discontinuity between the earth's atmosphere and outer space. This line of discontinuity is characterized by a large difference in infrared radiation between outer space and the earth which is considerably warmer and provides a large amount of infrared radiation as compared with space. Accordingly, the horizon represents a sharp line of discontinuity which provides an abrupt and sharp change of infrared radiation on either side which characteristic is utilized by scanning the detector in a conical pattern across the horizon and deriving electrical output signals marking that line of discontinuity. By using reference pulses as the scan passes predetermined points in a vehicle, the intervals between the horizon crossings and the reference pulses may be compared to produce an output signal providing information with respect to the attitude of the vehicle with respect to the horizon. Such horizon sensors have been very successful in space flight and have been used on many manned as well as unmanned satellites.
Originally, the motor which drove the optical scanner did so through a rotating assembly journaled on ball bearings on a detector lens mount. The scanning mechanism was ultimately improved by having the scanning axis of rotation of the motor shaft coincide with the optical axis of the scanning mechanism by providing a hollow shaft motor and mounting the detector inside the hollow shaft. The problem with this arrangement is that the sensor requires relatively large bearings to surround the hollow rotating motor shaft.